Although activating mutations of the KRAS oncogene are ubiquitous in pancreatic ductal adenocarcinoma (PDAC) and its earliest precursor lesions, pancreatic intraepithelial neoplasia (PanIN), animal model studies indicate that this mutation alone is not sufficient to initiate transformation. Instead, additional stimuli synergize with RAS o promote transformation, and this synergy is first evident in ductal reprogramming of the exocrine acinar cells from which PDAC arises. We hypothesize that the epigenetic program sustaining acinar differentiation constitutes as a novel tumor suppressive mechanism in these cells, and that mutual inhibitory interactions of acinar differentiation regulators with the RAS signaling pathway create a bistable switch between normal and transformed cell phenotypes. In particular, we propose that positive autoregulation of the transcription factor Ptf1a, a master regulator of acinar differentiation, is targeted for inhibition during PDAC initiation, and that restoration of its activity could restore transformed cells to a quiescent and differentiated state We will test this hypothesis in a mouse PDAC model, based on Cre- dependent expression of oncogenic KrasG12D in adult acinar cells, where we have discovered that genetic loss of Ptf1a dramatically enhances transformation. We propose two specific aims in this pilot study, combining hypothesis-driven and discovery-focused approaches: (1) determine whether Ptf1a downregulation is necessary and sufficient for RAS transformation of adult acinar cells; (2) identify potential epigenetic mechanisms for cross-inhibitory interactions of Ptf1a function and RAS signaling. These experiments take advantage of the PI and co-PI's expertise in mouse PDAC models and acinar gene regulation, and address a novel and potentially targetable mechanism for tumor initiation in this currently intractable disease.